1. Field of the Invention
The present invention relates to method and apparatus for providing Asymmetric Digital Subscriber Line (ADSL) service. More particularly, the present invention relates to method and apparatus for providing extended range broadband ADSL service for multiple users over a single twisted copper pair line.
2. Description of the Related Art
With rapidly increasing use of personal computers to access the internet, the market for broadband systems has been likewise developing to meet the necessary transmission bandwidth to reach residential and commercial sites. One such broadband system is a Digital Subscriber Line (DSL) system which is a high speed data transmission system that can transmit high speed data for applications such as video conferencing, fast internet access, interactive multimedia, on-line home banking, remote office or remote LAN applications. An ADSL system uses the existing twisted copper pair telephone lines to transmit digital signals in addition to the analog telephone signals between the users and the telephone companies, or in some circumstances, between the users and internet service providers (ISPs).
Compared with conventional analog modems that can accommodate a transmission rate of at most 56 kbps, ADSL systems can achieve access speeds that are approximately 150 times faster than the analog modems. A significant advantage of ADSL systems is that they can exist concurrently with the telephone signals such that a user connected to the internet via an ADSL modem can, at the same time, place or receive telephone calls using the same existing telephone line. This eliminates the need for installation of a second or third telephone line as would be the case when using an analog modem. In turn, this translates to substantial savings to the user, not to mention the much faster data access.
Demand from business or residential subscribers for higher speed internet access at lower costs has created a strong demand for DSL type services which provide digital access to the internet connections speed ranging from around 128 kbps up to 8 Mbps. Several different types of DSL systems exist. These include IDSL, HDSL, HDSL-2, SDSL, VDSL and RADSL, ADSL and ADSL Lite systems.
IDSL is an ISDN (Integrated Services Digital Network) based DSL system which uses 2B1Q line coding and typically supports duplex data transfer rates of 128 to 144 Kbps.
Also using 2B1Q line coding, HDSL refers to High Bit-Rate DSL which is generally used to deliver T1/E1 services, and supports full-duplex symmetric data transmission at rates up to 1.544 Mbps over 2 pairs of existing twisted copper pair telephone lines depending upon the transmission distance. While traditional T1 (E1 in Europe) requires repeaters at every 6,000 ft to boost the signal strength, an HDSL system can accommodate longer distances without the use of repeaters to support data transmission over approximately 12,000 kft.
HDSL-2 is a relatively new transmission technology that has recently become an ANSI standard for transmission of T1 (1.544 Mbps) rate data over a single copper pair. Unlike HDSL, HDSL-2 does not use 2B1Q encoding, but rather uses PAM-16coding approach which allows for the same transmission bandwidth as HDSL, but using half as much copper plant (for example, one twisted copper pair instead of two). Similar to HDSL-2, G.shdsl has been recently developed as an ITU standard for data transmission using PAM-16 coding scheme.
SDSL is a symmetric DSL system that is essentially a 2-wire implementation of a 2B1Q HDSL system. One implementation of symmetric DSL supports T1/E1 on a single twisted copper pair to a transmission distance of approximately 10,000 ft. A more common implementation of SDSL transmits sub-T1 speeds (for example, 384 kbps) to greater distances (up to 18,000 feet). Further, VDSL, Very High Bit-Rate DSL, has been proposed to support data rates exceeding 10 Mbps for comparatively shorter distances, such as 3,000 Kft.
ADSL, or Asymmetric DSL, is defined by its ability to provide much faster downstream transmission rates (of up to approximately 8 Mbps) compared to upstream data transmission rates (of up to approximately 1 Mbps). Since most residential computer users connected to the internet generally spend the majority of the time downloading information from the internet rather than uploading (or sending) information, this system is amenable to most users. Moreover, given its applicability for the residential mass-market, ADSL is projected to be the dominant form of DSL in the future.
Carrierless amplitude and phase modulation (CAP modulation) and Discrete Multitone modulation (DMT modulation) are two different modulation systems (or “line codes”) for ADSL currently on the market. More specifically, CAP modulation is a version of Quadrature Amplitude Modulation (QAM) in which incoming data modulates a single carrier that is then transmitted over a twisted copper pair telephone line and where the carrier itself is suppressed before transmission (i.e., the carrier contains no information, and thus, can be reconstructed at the receiving side). On the other hand, DMT modulation is a multicarrier modulation system in which incoming data is collected and then distributed over a large number of small individual carriers each of which uses a form of QAM modulation, and creates these channels using Discrete Fast-Fourier Transform. The ITU has specified DMT-based ADSL in its standards, and at the present time, the popularity of CAP-based ADSL appears to be on the decline.
ADSL-Lite, also known as “G.lite”, is an ITU standard (G.992.2) for providing a lower-speed, lower-power, widely interoperable form of ADSL-DMT. Its speeds are limited to 1.5 Mbps downstream and 512 kbps upstream. This limitation results in a simpler chipset, thus enabling low-power operation and wide interoperability among the DSL products of multiple vendors. A RADSL system is a Rate Adaptive DSL system that supports a wide range of DSL transmission rates. Originally, RADSL systems were ADSL systems that used CAP modulation rather than DMT modulation. However, today, DMT-based ADSL systems are also rate-adaptive, thus the terms “RADSL” and “ADSL” are generally synonymous.
The existing telephone plant includes twisted copper pairs extending from telephone central offices to subscribers. This is known as the outside plant (OSP) and may reach 25 miles or more. Statistically, 50% of subscribers reside within a 12,000-foot radius of the central office, 85% reside within 18,000 feet and approximately 99% of subscribers are within 40,000 feet.
The transmission distance of a RADSL/ADSL system varies according to the transmission bandwidth. At short distances (less than 8,000 feet), the ADSL systems can transmit at their full rates. However, at longer distances, the speeds available with ADSL systems decrease. Indeed, the longer the copper loop, the less bandwidth the system can support. As the loop length increases, a point is reached at which the system ceases to function altogether. This point is generally somewhere between 15,000 and 18,000 feet, depending on wire gauge and condition.
Existing ADSL systems and other DSL systems discussed above are generally limited to a transmission distance of up to 18,000 feet without the use of repeaters. It should be noted that the transmission distance is generally measured from the telephone company's central office to the customer premise equipment (CPE). In the United States, as discussed above, 18,000 feet covers less than 85% of subscribers. With higher transmission frequencies attenuating faster than at lower frequencies (for example, 784 kbps (SDSL) travels about 12,000 feet, while 256 kbps may travel up to 18,000 feet), and given a rapid rise in customer demands for higher speed internet access, customers are restricted ever more by their distance from the central office. In practice, an ADSL system at a transmission rate of 1.5 Mbps downstream may only work to a distance of 14,000 feet, serving approximately 60% of all potential subscribers.
The GDSL-8 system available from GoDigital Networks Corporation, the assignee of the present invention, provides multiple (for example, eight) telephone channels over a single copper pair, using a GDSL® Symmetric DSL transmission scheme transmitting at 544 kbps. Additional information related to the GDLS-8 system can be found in pending application Ser. No. 09/510,050 entitled Multiple Digital Subscriber Carrier System with Drop and Insert Repeater System, assigned to the assignee of the present application, and the disclosure of which is incorporated herein by reference in its entirety for all purposes. Another implementation of the GDSL system (the GDSL-12) is configured to run at 784 kbps with a payload of 12 telephone channels. The SDSL chips may transport anywhere from 160 kbps up to 2 Mbps in both directions simultaneously (symmetrical transmission) and are deployed in the telephone network for T1, ISDN, DAML, xDSL and other services.
As discussed above, ADSL service is generally provided to the user with the downstream rate greater than the upstream rate. For example, one ADSL service from Pacific Bell provides 784 kbps downstream and 384 kbps upstream. By slowing down the downstream channel, an asymmetric form of ADSL-DMT may be deployed to end-users. Additionally, US West currently offers some symmetric services to residential users that are not SDSL, but actually are ADSL-based. These services do not use 2B1Q transmission, but rather CAP modulation, and thus, fall into the category of ADSL systems discussed above.
The most common form of ADSL high speed service for internet access uses an ATM (asynchronous transfer mode) transport from the subscribers' terminal (such as personal computers), through the ADSL system, an into the telephone company's ATM switching network. In fact, the ITU standards for ADSL transmission call for ATM to be the transport protocol. An ATM network uses short, fixed-length cells with minimal headers, to allow calls to be routed at high speed by routing tables at each switch. The header of each ATM cell typically consists of 5 bytes and the cell information field includes 48 bytes, making a total cell length of 53 bytes. Accordingly, the header of each ATM cell is configured to provide two main routing functions—a Virtual Path Identifier (VPI) and a Virtual Call Identifier (VCI).
In a typical application, users send and receive almost no data most of the time during which the users (or user terminals) are connected to the internet. For internet access which is by far the most common application of ADSL, the system operates at a high data rate when downloading data in the form of pages or files. During the time that the users spend reading or reviewing the downloaded material, the connection line is again nearly idle. Indeed, a typical user may download and request data at a high rate for 5 to 30 minutes for every 24 hours. In particular, it is unlikely that more than one of the four ADSL users connected to an RTU will be downloading simultaneously, and further, it is more unlikely that more than two users will be doing so simultaneously.
An ADSL system requires a copper pair connected from the central office to each subscriber. While DMT-based repeaters may be developed to extend the range of ADSL transmission (i.e., repeating ADSL with additional ADSL spans), a separate circuit including the related hardware cost and a separate copper pair for each user is necessary.